The present invention relates generally to a method and means for accurately measuring the resistance of the cable shield and the resistance of earth grounds, power grounds, station grounds and ground bonds along telecommunications cables and particularly telephone cables in the field.
Good grounding of telephone cable shields and subscriber station grounds is required for safety and noise free reception of the voice or data being sent over the lines. When shielding is impaired, the technician must be able to accurately measure the ground resistance and shield resistance at cabinets, accesses and splices along the cables, to facilitate locating and repairing the poor shield bonding and grounding. He must also be able to accurately measure the subscriber station ground resistance to verify it is below the specified maximum of 25 ohms.
Internet Protocol Television, IPTV, and “triple play” services are now being deployed by AT&T and others over existing copper telephone pairs from in a Fiber-To-The-Node, FTTN system. In AT&T's system these nodes have “VRAD” gateways located next to existing cross-connect boxes already in the Telephone Outside Plant (telephone distribution cables). The VRAD is fiber-fed and provides high speed Internet Protocol data over the existing copper distribution cables in the “last mile” to the subscriber premise. In the present scheme the existing POTS, Plain Old Telephone Service, coming through the “feeder” cables from the Central Office to the cross-connect box, is sent through a splitter in the VRAD which combines the POTS with the VDSL, Very high speed Digital Subscriber Line. Then, both travel together to the subscriber.
IPTV is very sensitive to induced transients from nearby power lines and to short wave radio interference such as Ham Radio. To prevent such interference from entering the cable, the cable shields must be bonded through with low resistance connections from the VRAD to the subscriber. To test these shields for proper grounding, a test set must be able to read out ohms accurate down to one ohm. One also must be able to test along the cable without opening the splices for pair access. Thus, there is a need for a “Ground Resistance Test” to be used by maintenance technicians that reads directly in ohms and does not require pair access and a station ground verifier to be used by Premise Technicians.
The planned deployment of VoIP (Voice over Internet Protocol) over the VDSL links will allow the POTS lines from the central office “CO” to be disconnected. After this time, no office battery will be available for station ground verification or for the Ground Resistance Testing at the premise.
Various attempts have been made to overcome these problems. A conventional 2 terminal ohmmeter is not effective for verifying good shield grounding. The frequent presence of significant AC and DC voltages on the shields can render the ohms readings inaccurate, and it is difficult to be sure you have a good reference ground. The first lead of the ohmmeter is connected to the ground being measured, and the second lead is connected to a reference ground, frequently the cabinet ground or the shield of another cable or the MGN (Multi Ground Neutral of nearby power distribution) when available. A removable ground rod driven in the grass cannot be used for a two terminal measurement because a ground rod of manageable length will usually have resistance to earth above the 25 ohm minimum allowed reading for a Station Ground. Accurate ground resistance measurements down to 1 ohm are required to troubleshoot bonding along the feeder and distribution cables. If there is any resistance in the ground used as a reference, it will add to the resistance of the ground being measured. This added resistance frequently confuses the technician. For example 100 ohms in his reference ground can be falsely attributed to the shield resistance being measured, causing the technician to dig to open a buried splice that has perfectly adequate grounding. Earth salts can also interact with dissimilar metals in ground rods to create a battery affect that can cause conventional DC ohmmeters to peg out, making their readings unusable. As a result it is not recommended that a conventional ohmmeter be used to test shield grounding.
Multifunction Subscriber loop test sets such as the 3M 965 or Fluke 990 CopperPro include a Station Ground measurement based on a computation off the change in loop current when terminated Tip-Ring as compared to that when terminated Ring-Ground. The Ring-Ground termination eliminates half the pair loop resistance so a ballpark estimated ground resistance can be computed from the amount of increase of loop current. Unfortunately this test is not very accurate, and it is very inaccurate when the CO switch is a Lucent 5ESS with a floating ground battery. Most of the switches in the US are 5ESS so this test is normally not useful for measuring ground resistance.
Dedicated “Ground Resistance Testers” (three or four terminal ohmmeters) can give more accurate readings of ground resistance. These meters use two or three removable ground rods driven in the grass to allow measurement of the resistance of a Tested Ground without the resistance of the removable ground rods adding to the reading. These testers are more expensive than ohmmeters used by telephone technicians and are used infrequently. So only a small number of ground testers are purchased per work center. Substantial training is required as correct ground rod placement is required for accurate readings, and the meters are usually passed around without training, resulting in untrained personnel getting inaccurate readings. Thus, there is a need for an accurate ground resistance tester that is compatible with normal telephone technician troubleshooting procedures.
Ground resistance testers are available that use an inductive clamp to measure ground resistance. These testers have an advantage in that it is not necessary to disconnect shield ground bonds to get a reading. However, these testers are subject to problems similar to those of a two terminal ohmmeter in that any resistance in the reference ground adds to the measured resistance. Since cable shields are grounded at multiple points it is difficult to tell whether the measured resistance is in a ground being tested or in the connected cable shields, and it does not tell in which direction a measured open is located. Though this test is useful, it does not satisfy the need for a ground resistance test.
A method used by a few US Telephone Companies to test grounding applies current from Central Office battery to the Tested Ground and measures voltage potential fall (voltage drop) on that ground with respect to earth due to the current flow. A DC voltmeter is connected between a removable ground rod placed away from local grounding and the ring side of an idle working telephone pair. The voltmeter will thus display −48 to −52 Volts of office battery with respect to earth. Leaving the voltmeter connected, the ring conductor is then touched to the ground being verified. If the ground “kills” the voltage to below −1 Volt, the ground is deemed “good” (below 25 ohms to earth). The above 1 Volt value is based on current being sourced Ring to ground by the Central Office switch, typically 40 mA. 40 mA times 25 ohms=1 volt. This typical office current can vary from 20 mA to above 100 mA due to pair loop resistance and CO switch type so this test is very inaccurate and cannot measure below 25 ohms. Measurements to one ohm are needed to troubleshoot shield grounding and bonding.
Thus a means for simply measuring ground resistance is needed that is accurate, less expensive, more intuitive, and requires little training. This unit would measure ground resistance or verify station grounds and shield grounding at any access along the cable. The ohms readout could be used by maintenance technicians and a go/no-go indication could be used by entry level premise technicians wiring homes for VDSL and IPTV.
Measurement of ground resistance with a classic two terminal ohmmeter will frequently give erroneous readings, which cause lost time when looking for open shields. Commercially available Ground Resistance Testers require the use of two removable ground rods, and accurate measurements depend on the correct location of the two rods. Operation of these testers is beyond the scope of the training a typical telephone technician receives. Therefore, there is also a need for a test that falls within the normal training of telephone technicians, is easy to use, and can be included in his multi function test set.